1. Field of the Invention
This invention relates to a system for measuring a quantity of light reaching the system and also controlling exposure corresponding to a sensed scene to be photographed and more particularly relates to a circuit arrangement for digitized calculation for obtaining a measured quantity of light.
2. Description of the Prior Art
In recent years, systems such as an electronic still camera including a system for measuring a quantity of light for a scene to be photographed over relatively wide range of the measuring have been proposed (see laid-open Japanese patent application, publication No. 60-43979). In such a system, a plurality of amplifiers are provided for changing over the gain according to the level of an output signal produced by a photoelectric conversion device thereby extending the dynamic range for the measurement. One of the amplifiers is selectively connected to the photoelectric conversion device according to the output level of the output signal of the photoelectric conversion device. In the above-mentioned system, much many more amplifiers are needed for obtaining a wide range of the measurement with sufficient resolution, therefore the circuit arranged in the system comes is out very complicated and extravagant aspect.
On the other hand, an exposure control device employing a digitalized technique utilizing a binary counter has been proposed (see published Japanese patent application, publication No. 57-58647). In this device, exposure time is controlled utilizing counting operation in a reversible counter and charging operation of a capacitor. The exposure time is determined by the sum of a time for reverse counting from a preset number in the reversible counter and a time corresponding to a charged-up voltage across the capacitor. Since the device is so arranged that the repetitive frequency for pulses to be counted in the reversible counter is set as relatively low, no large aspect counter is needed for covering wide range of the exposure time. On the contrary, each count number in the reversible counter necessarily represents relatively long time, and a separately provided special capacitor charging operation should be needed for representing a time shorter than the time represented by each count number in the reversible counter so as to obtain a sufficient resolution for determining an exposure time. Such devices employing capacitor charging or discharging operation to represent a time period cannot perform so accurate measurement or control of time, as is generally known.
While a camera capable of taking electronic still pictures employing an auxiliary sensor for measuring the intensity of light and controlling exposure has been proposed (see laid-open Japanese patent application, publication No. 58-156274), the camera has no means for effective compensation for exposure.